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The Physiology of the Senses
Lecture 10 - Balance
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Contents Objectives ........................................................................................................................ 1 The sense of balance originates in the labyrinth. ............................................................. 2 The auditory and vestibular systems have a common origin. .......................................... 2 The vestibular system has two parts. ............................................................................... 3 The Anatomy of the Otolith Organs ................................................................................ 3 What is the functional anatomy of the semicircular canals?............................................ 6 The Vestibular Ocular Reflex (VOR) .............................................................................. 7 Why do we get dizzy? ...................................................................................................... 8 When does the VOR gain need to be adjusted? ............................................................... 9 What adjusts the VOR?.................................................................................................. 10 See problems and answers posted on ........................................................................... 11 Objectives
1) Specify which anatomical aspects of otolith organs and vestibular canals make
them best suited for detecting different types of motion.
2) Explain how a particular translational direction is coded by hair cells in the
otolith organs.
3) Given an arbitrary head rotation, predict which vestibular canal is most active.
4) List the synapses in the vestibular ocular reflex.
5) Explain how the optokinetic response helps prevent dizziness and also causes it.
6) Contrast which parts of the cerebellar circuits act as teachers and which the
students.
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The sense of balance originates in the labyrinth.
The bony labyrinth is a convoluted system of tunnels
in the skull that contains the sensors for hearing and balance.
The inside of these tunnels is lined with a membrane. The
space between the bone and the membrane (Figure 10.1)
contains perilymph a fluid somewhat similar to extracellular
fluid. Endolymph, a fluid
Figure 10. 1
The Labyrinth
similar to intracellular fluid (i.e. and Vestibular System Inside the
high K+, low Na+) fills the
bony labyrinth (grey) one finds the
inside of the membrane and
perilymph (yellow) and inside the
surrounds the balance receptors. membrane, the endolymph (pink).
The sensors for hearing are
close to those for balance because they have a common
origin.
The common origin is the lateral line organ that first evolved in early fish. This organ
consists of tubes that lie along the fish’s side. As the fish swims, water flows through these
tubes and across the sensory cells, which have hair-like projections that are bent by the water.
Fluid movement in the tubes is caused by
1. waves produced by some noise in the water (the precursor of the auditory system) and
2. the fish's own motion (the precursor of balance or the vestibular system).
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The vestibular system has two parts.
The vestibular system has two parts, the otolith organs and the semicircular canals.
Each has a different function.
Otolith Organs
The otolith organs have two
functions:
Semicircular Canals
The canals detect the head’s
rotation (turning motion).
1. The otoliths
sense the head’s
linear
acceleration
(motion in a
straight line).
They sense how
quickly you are accelerating forward
or backward, left or right, or up or
down.
2. They are also able to
sense the head’s
position relative to
gravity. These are the
organs that tell us
whether we are upside
down or right side up.
The Anatomy of the Otolith Organs
Inside the otolith organs are two sacs called the utricle
and the saccule. On the inside of each a portion of the sac is
thickened and called the macula (blue and green ovals in Figure
10.2). The macula contains hair cells innervated by neurons of
the 8th nerve.
The hair cells project into a gel. Calcium carbonate
crystals (ear stones) are embedded in this gel. The purpose of the
stones is to give the gel extra mass.
The thickest and longest of hairs on hair cells is the
kinocilium.
Figure 10. 2
The
otolith organs consist of the
utricle and saccule.
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How is motion transduced into neural firing?
The steps are:
1) As in auditory hair cells, motion bends the hairs.
2) Endolymph, high in K+ and low in Na+ surrounds the hairs. Endolymph has a
+80 mv charge with respect to the hair
cell.
3) The filament between adjacent hairs opens
ion channels. The endolymph’s positive
voltage pushes K+ into the negatively
charged hair cell (Figure 10.3).
4) The hair cell depolarizes, releasing
neurotransmitter.
5) There is an increase in the frequency of
AP's in the 8th nerve afferent.
Figure 10. 3
The Hair Cell’s
Hairs and Filaments When the flap is
opened, K+, potassium, enters the cell.
What bends the hairs?
When the hairs are undisturbed, the vestibular
afferents have a resting firing rate of about 100 action
potentials per second (Figure 10.4).
When the head moves, the inertia of the crystals bends
the hair cells in the opposite direction. Bending all the hairs
towards the tallest hair, the kinocilium, opens the ion channel
and depolarizes the cell, inducing an increase in action
potential frequency in the 8th nerve afferents.
Bending away from the kinocilium closes the ion
channel and causes hyperpolarization and reduces the action
potential frequency.
Figure 10. 4
The Effects of Motion on
Hair Cells Bending the hairs toward the tallest
hair increases the potential inside the cell and the
rate of action potentials produced. Bending in
the opposite direction produces the opposite
effects.
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In addition to head movements, gravity "pulls" on
the stone crystals. When head position changes, the
direction of this gravitational "pull" changes, telling you
that your head is tilted (Figure 10.5).
Figure 10. 5
The weight of the
crystals pulls them down, as well as
the attached hairs, activating the hair
cell.
Within the macula of the utricle
and saccule (Figure 10.6), the kinocilium
of the hair cells are oriented in all
possible directions on the surface of the
macula (the location of kinocilium is
indicated by the red arrows). The
direction of linear acceleration or gravity is
determined by which hair cells are most
bent toward the kinocilium.
With the head upright, the macula
of the utricle (green) is in the horizontal
plane and senses left/right and
forward/backward translations.
The macula of the saccule (blue) is
on the side and senses translations in the
vertical plane (up/down and
forward/backward).
Figure 10. 6
The Orientation of the Hairs in the Otoliths
The arrow tip indicates the orientation of the kinocilium in the hair
cells of the utricle (green) and saccule (blue). The box shows their
orientation in the head.
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The semicircular canals sense head rotation.
There are three canals on each side of your head (Figure 10.7A). One is approximately
horizontal (h), and the other two, the anterior (a) and posterior (p), are vertical. All three are
about perpendicular to each other and thus form three sides of a cube. Within the canals are
endolymph-filled semicircular ducts and each has a swelling called the ampulla (Figure 10.7B).
A pliable membrane called the cupula seals the inner diameter of the ampulla. The hairs of the
hair cells project into the cupula.
Figure 10. 7
The Orientation of the Canals in the Head A: the horizontal (h), anterior (a) and posterior (p) canals have
different orientations (indicated by arrows). The top is towards the nose and the rightmost canals are on the right side of the head.
B: the ampulla (shown inside the blue dotted circle contains the cupula and the hair cells. The latter become deflected during a
head rotation.
How do the canals detect angular acceleration of the head?
When there is a change in speed of head rotation, the endolymph fluid lags behind,
because of inertia, pushing on and distorting the cupula. The bending of the hair cell hairs
causes increase or decrease of the hair cell potential, depending on whether bending occurs
towards or away from the kinocilium.
How do the canals compute direction of head rotation?
Since there are three canals on each side of the head and they are roughly perpendicular
to each other, the activity in the canals decomposes all rotation into three components, so much
to the right, so much downward, and so much clockwise. Also, the canals are arranged such that
each canal has a partner on the other side of the head. When one partner’s hair cell potential is
increased, the other’s is decreased. This is called a push-pull organization. When the rotation
is in the plane of a canal push-pull pair, the potential of this pair’s cells are increased or
decreased while the other four canals show no change. When the head rotates rightward in the
plane of the horizontal canals, the potential increases in the horizontal canal on the right side of
the head and decreases in the left. No change in potential occurs in the other four canals.
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The anterior canal on one side and the posterior on the other also form push-pull pairs
When you tip your head forward and to the left ear, in the plane of the left anterior canal, this
canal’s hair cells increase in potential while those of the right posterior canal decrease. No other
canal changes its activity. When you tip your head forward and to the right ear, the right anterior
canal’s hair cell potential increases while those of the left posterior decreases.
The Vestibular Ocular Reflex (VOR)
The otoliths and canals activate many postural reflexes. These connect to muscles in your
legs, trunk and arms and keep you upright. Another key reflex is one that turns the eyes, the
vestibular ocular reflex (VOR). The important function of the VOR is to stabilize the retinal
image during rotations of the head. To maintain a clear image, requires keeping the eye still in
space in spite of any head translation or rotation. For example, when the head rotates with a
certain speed and direction, the eyes must rotate with the same speed but in the opposite
direction (Figure 10.8). The ratio of the eye and head rotations is called the gain of the VOR.
The ideal gain is -1. This gain keeps the image of the world stationary on the retina. Many of the
newer smartphones use Optical Image Stabilization for the same reason.
Figure 10. 8
Rotations of the eye
should cancel those of the head. If the
eye rotation (red) in the head is the
opposite of that of the head (blue) the
eye will stay still (black flat line).
Explain the neural mechanism for a horizontal VOR.
When the head rotates rightward, the following occurs (Figure 10.9):
1) The right horizontal canal hair cells depolarize (potential increases) while those of the
left hyperpolarize (potential decrease).
2) The right vestibular afferent activity increases,
while activity of the left decreases.
3) The right vestibular nucleus’ activity increases
while that in the left decreases.
4)
In the cranial nerve (motoneurons to extraocular
muscles), neurons in the left 6th and right 3rd
nerve nuclei fire at a higher frequency.
5) Those in the left 3rd and right 6th nerve nuclei
fire at a lower frequency.
6) The left lateral rectus (lr) extraocular muscle and
the right medial (mr) rectus contract.
7) The left medial rectus and the right lateral rectus
relax.
8) Both eyes rotate leftward.
Notice the push-pull organization, an increase on
one side is accompanied by a decrease on the other.
Figure 10. 9
The Horizontal VOR Turning the
head to right (green) activates the right horizontal canal,
the right vestibular nucleus ((vn), the motoneurons in the
left 6th nerve nucleus (6th), left lateral rectus (lr), the right
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medial rectus (mr), and both eyes turn to the left. The
mirror images neurons (orange) show an inhibition of
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activity when turning right.
Why do we get dizzy?
During normal head rotations, the eye rotates opposite
to the head, canceling the motion of the head. This stabilizes
the image of the world on the retina (Figure 10.10).
When you turn your head, the cupula in you
Figure 10. 10
The Normal Vestibular Ocular
canal becomes deflected, signalling that you are
Response Head motion in one direction is canceled
turning. During a prolonged head rotation (20sec or
by eye rotation in the other, resulting in a
more), the elasticity of the cupula gradually restores
stationary image on the retina.
it to its upright position. The drive to the VOR stops
and, if your eyes are closed, you falsely sense that
you are stationary. If you then open your eyes, you see that
the world moving and you feel dizzy (Figure 10.11).
Visual input, on its own, can drive the VOR, the
optokinetic response (OKR), but the OKR takes time to build
up. When the visual scene on your retina starts to move (retinal slip)
the OKR kicks in producing a rotation of the eyes in the
opposite direction. Thus an initial slip of the world in the
eye’s view is followed by a stabilized image. This initial
retinal slip can elicit a false perception of motion. For
example when you look out a car window and see an adjacent
car start to move you often sense yourself move. The visual
input comes from MSTd, which senses optic flow, Figure 10. 11
Prolonged rotations make you dizzy.
the visual motion produced when you move. The
After 20 sec of rotation, the cupula springs back to its
signal from vision and the cupula is combined in
upright position signalling that you are stationary. If you
the vestibular nuclei (Figure 10.12) and then sent
now open your eyes you will see that in fact you are still
to the thalamus. The thalamus then projects to the rotating and you will feel dizzy. Top red dashed line:
primary somatosensory cortex where activity
activity of hair cells. Blue line: head rotation.
elicits the subjective sense of self-motion.
Many do not become very dizzy during a prolonged
rotation if they keep their eyes opened and fixate on
stationary objects around them. This is because the visual
input compensates for loss of vestibular drive. Visual input
builds up as the vestibular input dies away, as the cupula is
restored to its normal position. The net result is that the eye's
view of the world remains stable (Figure 10.12). A ballet
dancer avoids becoming g dizzy by spotting. Although the
body spins continuously, the head rotates more rapidly and then is briefly still. This prevents
adaptation of the cupula.
Motion sickness occurs when the two signals are in conflict. Suppose you are inside the
cabin of a boat during a storm. Your vestibular afferents
Visual and vestibular
sense that you are moving. Because you and the cabin are Figure 10. 12
signals
combine
to
compute
a correct
moving together, the visual system senses that you are
estimate of head velocity. Green dashed
still. The visual and vestibular signals are in conflict. To
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visual input of optic flow. Red solid: vestibular
estimate of head velocity. Black dotted: The
combined correct estimate.Revised 18/11/2015
avoid motion sickness the best solution is to go out on the deck and look at the horizon. Here
both signal the same motion. Similarly, the best way to get motion sickness in a car is to sit in
the backseat and read. The best way of avoiding motion sickness is to sit in the front seat and
look out the front window.
One theory for the feeling of nausea that sometimes accompanies dizziness is that the
brain interprets this conflict as poisoning and responds by eliciting vomiting to clear the poison.
More Cases of Feeling Dizzy
You can imagine that, as your life goes by, some neurons involved in the VOR may die
or malfunction or the eye muscle strength may decrease. This would lower the VOR gain.
If you change the prescription on your glasses (with an increase or decrease in
magnification), the change in optics changes the VOR gain you need. You initially may feel
dizzy because of the retinal slip when wearing your glasses.
If you have an abnormal vestibular input you may experience the following disorders:
1) difficulty reading signs while walking because the VOR fails to stabilize the eyes and
2) difficulty standing with your eyes closed because the vestibular spinal reflexes, on
their own, fail to assist posture correctly.
Most often, these disorders only produce a transient defect because the VOR is
continuously adjusted and fine-tuned.
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What adjusts the VOR?
The VOR is the combination of three pathways (Figure 10.13):
1) the direct pathway through the vestibular nucleus to the eye muscles.
2) the indirect pathway through the cerebellum. This involves the mossy fibers, then
parallel fibers, and finally Purkinje cells which inhibit the vestibular nucleus.
3) the teacher, via visual input through the climbing fibers, changes the strength of
the connections between parallel fibers and Purkinje cells, the indirect pathway.
Figure 10. 13
The Cerebellar Repair Shop A: the direct pathway (red). B: the indirect pathway (green)
through the cerebellum subtracts a correction. C: the climbing fiber input modifies the strength of the indirect
pathway.
The VOR gain is determined by the difference between the direct and indirect paths. The
cerebellum's task is to keep this difference optimal in spite of all the changes that may occur to
the various parts of the direct VOR.
What teaches the cerebellum?
When VOR is not working properly (e.g. the eye is not rotating enough or too much) a
slip of the image is detected by the retina and sent to the cerebellum via the inferior olive
climbing fiber input. This is the teacher’s input, which semi-permanently alters the synapses of
the students; that of all concurrently activated parallel fibers. This increases or decreases the
cerebellar inhibition of the vestibular nucleus. When the activity of the vestibular nucleus is just
correct, the retinal slip stops and the teacher is silenced. The cerebellum acts like a repair shop.
It makes similar re-adjustments to all our reflexes.
Alcohol and many drugs affect the function of the brain and this repair shop. It is thus
not surprising that when the repair shop malfunctions, the VOR becomes uncalibrated, and one
feels dizzy.
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See problems and answers posted on
http://www.tutis.ca/Senses/L9Auditory/L9AuditoryProb.swf
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